Electron microscopes including transmission electron microscopes (TEM) and scanning electron microscopes (SEM) have acquired ever increasing resolutions in terms of space, time, and energy as a basic tool for studies in a wide range of fields from material science to biology. Improved resolutions require a brighter point electron source for an electron microscope, and a stable emission electron beam with high time coherence needs to be generated. CFE has the highest brightness and time coherence compared to other electron sources such as Schottky emitters and thermionic emitters.
In CFE, the brightness of the electron source is related to emission current density. This can be represented by the simplified Fowler-Nordheim equation of the formJ=c1F2/φexp(−c2φ3/2/F)  (1),where the variables F and φ are the local electric field strength, and the work function of the emitter material, respectively. The symbols c1 and c2 can be treated as constants under the actual operating conditions. The time coherence of CFE can be characterized as the spread of emitter energy, as can be represented by the following equation.ΔE=c3F/φ1/2  (2),where c3 is a constant. It follows from this that emission current density J can be regarded as a function of the two independent variables φ and ΔE, as represented by the following formula (3) obtained by combining the equations (1) and (2)J=c4ΔE2exp(−c5f/ΔE)  (3),where c4 and c5 are complex constants of constant parts that occur in the process of deriving the equation. It follows from equation (3) that smaller work functions produce higher emission current densities and smaller energy spreads at the same time.
Tungsten (W), a conventional CFE material, has a work function in excess of 4.5 eV, though it varies with the crystal plane. LaB6, having a (100) plane work function of 2.6 eV, represents a more desirable emitter material. LaB6 can produce very high brightness, and has been used as an alternative material of the W thermionic emitter. This is due to the fact that in addition to the small work function, LaB6 has other advantages such as high conductivity, high mechanical strength, and high melting point. The thermionic emitter operates at temperatures above 1500° C. Here, all the residual gas in a vacuum is the result of desorption from the emitter surface, and the LaB6 surface remains clean. Another advantage of the high-temperature heating is that the LaB6 end surface is La-terminated. This is important because the work function is small only when the LaB6 surface is La-terminated. The B-terminated LaB6 surface, on the other hand, has a large work function. However, the clean La-terminated LaB6 surface is rapidly contaminated when the LaB6 is operated at temperatures below 900° C., or in a cold field emission mode that involves temperatures below room temperature. In this case, the emission current decreases as much as 90% in 5 minutes, when the practical electron source for electron microscopes requires at least several hours of operation time.
Another problem of the LaB6 emitter operating at low temperature is that the emitter has a B-terminated surface. This is problematic because the characteristic advantage of LaB6, specifically the small work function of the La-terminated surface cannot be exploited when the LaB6 emitter based on the related art is used in the cold field emission mode.
This is specifically as follows. The work function of LaB6 is determined by the terminal surface atoms, specifically the atoms appearing on the crystal surface. The work function is greater for the B-terminated surface, and smaller for the La-terminated surface. The (100) crystal plane of a LaB6 nanowire produced in the related art is terminated on the B atoms, for example, as described in Non-Patent Literature 2, and the (100) plane of such a LaB6 nanowire has a large work function. When used as an emitter, such a LaB6 nanowire emits electrons from the (110)/(210) plane having a relatively small work function. Because the LaB6 nanowire extends in a <100> direction, electrons are not released from the B-terminated (100) plane of a large work function, but are emitted from the surrounding (110)/(210) plane. The emission pattern thus has a dark central portion, with bright spots distributed in the peripheral portions (see FIG. 5(b)). A nanowire that extends in a (210) direction produces an emission pattern appearing bright in the central portion; however, the surrounding (110) region, and other (210) region also appear bright. In an ideal point electron source, emission must occur only from one location at the central portion of the emission pattern. The best form of a cold electron source, then, is when a LaB6 nanowire has a La-terminated (100) plane at the tip. These are described in, for example, Patent Literatures 1 and 2, and Non-Patent Literatures 1 and 2. Readers are requested to refer to these documents, as necessary.